FFI docs: state how to expose non-function symbols (or state if unsupported)

[pnkfelix]

From reading rust.md and tutorial-ffi.md, I did not see how to expose a global variable from a library. (E.g. I've been poking at hooking into pdcurses / ncurses, and ncurses in some platforms uses global variables to expose curscr/newscr/stdscr).

As far as I can tell, one can do this via the syntax you would expect, namely:

pub extern {
    static curscr: *WINDOW;
    ....

    unsafe fn addch (_:chtype) -> c_int;
    ...
}

But we should be more up front about whether this is the official syntax, or whether I am just getting lucky here in the short-term and that one should not expect linkage to non-functions to work in the long-term.

[emberian]

@alexcrichton can you clarify this?

[alexcrichton]

When I was adding static mut I also "got lucky" in doing that exact syntax. At least for static mut, that's the intended syntax for interfacing with an external C library.

If this isn't documented though, I do agree that it should be documented. I'll see if I can't write up a section at least on static mut and maybe static in general inside the extern block.

[alexcrichton]

Closed in 1b10391